Shifts in rhizosphere soil microorganisms of dominant plants' response to climate change profoundly impact mountain soil ecosystem multifunctionality; relatively little is known about the relationship between them and how they depend on long-term environmental drivers. Here, we conducted analyses of rhizosphere microbial altitudinal pattern, community assembly, and co-occurrence network of 6 dominant plants in six typical vegetation zones ranging from 1350 to 2900 m (a.s.l.) in Helan Mountains by absolute quantitative sequencing technology, and finally related the microbiomes to root zone soil multifunctionality ('soil multifunctionality' hereafter), the environmental dependence of the relationship was explored. It was found that the altitudinal pattern of rhizosphere soil bacterial and fungal diversities differed significantly. Higher co-occurrence and more potential interactions of Stipa breviflora and Carex coninux were found at the lowest and highest altitudes. Bacterial α diversity, the identity of some dominant bacterial and fungal taxa, had significant positive or negative effects on soil multifunctionality. The effect sizes of positive effects of microbial diversity on soil multifunctionality were greater than those of negative effects. These results indicated that the balance of positive and negative effects of microbes determines the impact of microbial diversity on soil multifunctionality. As the number of microbes at the phylum level increases, there will be a net gain in soil multifunctionality. Our study reveals that geographical and climatic factors can directly or modulate the effects of soil properties on rhizosphere microbial diversity, thereby affecting the driving effect of microbial diversity on soil multifunctionality, and points to the rhizosphere bacterial diversity rather than the fungi being strongly associated with soil multifunctionality. This work has important ecological implications for predicting how multiple environment-plant-soil-microorganisms interactions in mountain ecosystems will respond to future climate change.

C. korshinskii is one of the most widely-planted sand-fixing legumes in northwest China and exploring its rhizosphere microbiome is of great ecological importance. However, the effect of long-term sand fixation on the composition, diversity, and underlying functions of microbes in the C. korshinskii rhizosphere in dryland ecosystems remain unclear. Here, we performed high-throughput sequencing using a 16S rRNA (absolute quantification) and bacterial functional annotation of prokaryotic taxa (FAPROTAX) analysis and an ITS (relative quantification) and fungal functional guild (FUNGuild) analysis to investigate the C. korshinskii rhizosphere microbiome and metabolic functional groups at different sand-fixing ages (six years, CK6; twelve years, CK12; and eighteen years, CK18) and determined the physicochemical properties of the rhizosphere soil. Results showed that the key bacterial taxa of the rhizosphere were significantly more abundant in CK18 than in CK12 and CK6 at the phylum-class-genus level, and that fungal Glomeromycota was also significantly more abundant in the CK18 rhizosphere compared to CK12 and CK6. Among these bacterial taxa, the enrichment effect of key, functional, genus-level species of bacteria was the most obvious, including Rhizobium, Ensifer, Neorhizobium, Mesorhizobium, Streptomyces, Sphingomonas, and Flavobacterium, which are N-fixing and/or phosphate-solubilizing groups. The significant improvement seen in the physicochemical properties of the CK18 rhizosphere soil, including the higher total nitrogen (TN), available nitrogen (AN), pH, electrical conductivity (EC), higher N:P ratio, and lower C:N ratio, all demonstrated the relationship between the rhizosphere microbes and soil carbon (C) and nitrogen (N) cycling. A redundancy analysis (RDA) of different taxonomic levels indicated a close positive relationship between rhizosphere microbes and AN. In addition, the functional groups of the C. korshinskii rhizosphere bacteria were closely related to soil AN and were mainly composed of chemoheterotrophy and aerobic chemoheterotrophy. A Spearman correlation analysis revealed that these functional groups were mainly identified from bacterial Actinobacteria, Proteobacteria, Verrucomicrobia, Bacteroidetes, and fungal Glomeromycota. Our study provides evidence that the rhizosphere microbes of C. korshinskii are closely related to the accumulation of N in the restoration of desert ecosystems, and that the ecological functional processes they are involved in mainly involve C and N cycles, which play an important role in desertification reversal.

Desertification leads to the extreme fragility of ecosystems and seriously threatens ecosystem functioning in desert areas. The planting of xerophytes, especially leguminous shrubs, is an effective and common means to reverse desertification. Soil microorganisms play a crucial role in nutrient cycling and energy flow in ecosystems. However, the effects of introducing leguminous shrubs on soil microbial diversity and the relevant mechanisms are not clear. Here, we employed the high-throughput absolute quantification 16S rRNA sequencing method to analyze the diversity of soil bacteria in sand-fixing areas of mixed shrublands with three combinations of shrubs, i.e., C. korshinskii × Corethrodendron scoparium (CaKCoS), C. korshinskii × Calligonum mongolicum (CaKCaM), and C. scoparium × C. mongolicum (CoSCaM), in the south of the Mu Us Sandy Land, China. This area suffered from moving dunes 20 years ago, but after introducing these shrubs to fix the dunes, the ecosystem was restored. Additionally, the effects of soil physicochemical properties on soil bacterial composition and diversity were analyzed with redundancy analysis (RDA) and structural equation modeling (SEM). It was found that the Shannon index of soil bacteria in CaKCoS was significantly higher than that in CaKCaM and CoSCaM, and the abundance of the dominant phyla, including Actinobacteria, Proteobacteria, Acidobacteria, Chloroflexi, Planctomycetes, Thaumarchaeota, Armatimonadetes, candidate_division_WPS-1, and Nitrospirae, increased significantly in CaKCoS and CaKCaM compared to that in CoSCaM. RDA showed that the majority of soil properties, such as total nitrogen (TN), available potassium (AK), N:P ratio, soil moisture (SM), and available phosphorus (AP), were important soil environmental factors affecting the abundance of the dominant phyla, and RDA1 and RDA2 accounted for 56.66% and 2.35% of the total variation, respectively. SEM showed that the soil bacterial α-diversity was positively affected by the soil organic carbon (SOC), N:P ratio, and total phosphorus (TP). Moreover, CaKCoS had higher SM, total carbon (TC), total potassium (TK), and AP than CaKCaM and CoSCaM. Collectively, these results highlight a conceptual framework in which the combination of leguminous shrubs can effectively drive soil bacterial diversity by improving soil physicochemical properties and maintaining ecosystem functioning during desertification reversal. Graphical Abstract

The soil microbiome plays a vital role in maintaining soil nutrient levels and ecological stoichiometry balance. However, the relationships between rhizosphere microbiomes and soil ecological stoichiometric characteristics, including organic carbon (SC), total nitrogen (SN), total phosphorus (SP), and their ratios, remain poorly understood. Here, we used a temperate mountain ecosystem as a natural laboratory along a ~ 2190 m elevational gradient spanning a desert steppe-alpine meadow transition. We investigated rhizosphere microbiomes from 20 dominant plant populations across 17 sites by integrating environmental factors, microbial community structure, functional genes, microbial biomass, and ectorhizosphere soil stoichiometric characteristics. Ectorhizosphere soil stoichiometric characteristics were significantly associated with microbial biomass stoichiometric characteristics, rhizosphere community composition, and C-, N-, and P-cycling genes, with functional genes emerging as the strongest predictors. Structural equation modeling further identified the composition and diversity of functional genes as key drivers of soil stoichiometric characteristics. Geographic and edaphic factors exerted primarily direct effects, whereas climatic influences were indirect and mediated through the rhizosphere microbiome. These findings highlight the rhizosphere microbiome as a critical biological filter linking climate to soil nutrient stoichiometry at the root-soil interface.

Desertification reversal is globally significant for the sustainable development of land resources. However, the mechanisms of desertification reversal at the level of plant community are still unclear. We hypothesized that desertification reversal has clear effects on plant community composition, plant functional types (PFTs), and other vegetation characteristics, including plant diversity and biomass, and their changes in the early stages of reversal are more dramatic than in later stages. We investigated the vegetation of four to five different stages of desertification reversal at each of seven large study sites in southwestern Mu Us Sandy Land, China. The results show that the dominant species in very severe desertification areas were replaced by perennial grasses in potential desertification areas. The importance values of annual forbs and perennial sub-shrubs decreased dramatically (from 42.59 and 32.98 to 22.13 and 5.54, respectively), whereas those of perennial grasses and perennial forbs increased prominently (from 13.26 and 2.71 to 53.94 and 11.79, respectively) with the reversal of desertification. Desertification reversal increased the complexity of plant community composition by increasing plant species in each PFT, and C₃ plants replaced C₄ plants to become the dominant PFT with reversal. Plant species richness and species diversity rose overall, and aboveground plant biomass significantly (p < 0.05) increased with the reversal of desertification. Most vegetation characteristics changed more strikingly in the early stages of desertification reversal than in later stages. Our results indicate that the type and composition of the plant community were dramatically affected by desertification reversal. Anthropogenic measures are more applicable to being employed in early stages than in later stages, and Amaranthaceae C₄ plants are suggested to be planted in mobile dunes for the acceleration of desertification reversal. This study is useful for designing strategies of land management and ecological restoration in arid and semiarid regions.